Receiving test system is generally used in calibration of step attenuators. When the object to be tested relates to calibration of wideband signals, the calibration instrument must be tuned with the tested frequency and re-calibrated for each test point. Once the instrument is tuned in the same frequency, it requires additional calibration when the calibrated attenuation exceeds the attenuation prescribed by the instrument. Therefore, in an actual operation of calibrating a step attenuator, the calibration instrument constantly and repetitively performs self calibration that results in great time consumption. Testing may also be prohibited due to the limitation of calibration in an alternative condition that the attenuation of the object to be tested exceeds the total attenuation of the calibration instrument. Thus, the calibration instruments need only calibrate once by utilizing the present invention without suffering from the limitation of the total attenuation of the object to be tested.